• Down syndrome;
  • Robertsonian translocations;
  • mosaics;
  • prenatal diagnosis


This study describes the characteristics of karyotypes leading to phenotypic Down syndrome (trisomy 21) in 29,256 cases diagnosed between 1989 and 2009 in England and Wales included in the National Down Syndrome Cytogenetic Register (NDSCR). The frequency of occurrence of the different karyotypes, proportions diagnosed prenatally, sex ratios, mean maternal age, and proportions of mothers with recurrences were analyzed. Nearly 97% of all cases were free trisomy 21; 2.9% contributory trisomy 21, 0.3% double or triple aneuploidies; 1% of all were mosaics. Mean maternal age of free trisomy 21 cases was 35 years, 54% were male, and 1% of mothers had recurrences. Free trisomy 21 mosaics had a lower mean maternal age (33 years), a lower proportion of males (39.5%), and 2.5% of mothers had recurrences. The majority of contributory translocations were Robertsonian or rea (21;21). Their mothers were younger, particularly those of Robertsonian translocations (28 years). Of the Robertsonian der (14;21) translocations of known parental origin, 54% were de novo, 41% maternal and 5% paternal and 15.8% of mothers of those of maternal origin had recurrences. Multiple aneuploidies have the highest proportion of males (67%), highest proportion of mosaics (40%), a mean maternal age of 37 years, and no mothers had a recurrence. The size of this national register allowed the frequency of occurrence of the rarer karyotypes of Down syndrome to be estimated and their epidemiology described. © 2012 Wiley Periodicals, Inc.